The technology behind flat-panel displays, such as liquid crystal or plasma displays, has advanced to the stage where a single display can be economically manufactured to about the screen size of a modest domestic television set. To increase the display size of a single-unit display beyond this level introduces dramatically greater costs, lower manufacturing yields and other significant technical problems.
To provide larger displays, therefore, a hybrid technology has been developed whereby multiple smaller rectangular displays are tessellated to form the required overall size. For example, a 2×2 tessellated array of 15 inch diagonal displays, with appropriate addressing electronics to route pixel information to the appropriate sub-display, would provide a 30 inch diagonal display.
A drawback of this type of arrangement is that the active area of an individual display, that is to say, the area of the front face of the display on which pixel information is displayed, does not extend to the very edge of the physical area of the display. The technologies used, whether plasma, liquid crystal or other, require a small border around the edge of the active display area to provide interconnections to the individual pixel elements and to seal the rear to the front substrate. This border can be as small as a few millimetres, but still causes unsightly dark bands across a tessellated display.
Various solutions have been proposed to this problem, most of which rely on bulk optic or fibre optic image guides to translate or expand the image generated at the active area of the individual sub-displays.
For example, U.S. Pat. No. 4,139,261 (Hilsum) uses a wedge structure image guide formed of a bundle of optical fibres to expand the image generated by a panel display so that by abutting the expanded images, the gap between two adjacent panels, formed of the two panels' border regions, is not visible. The input end of each fibre is the same size or less than a pixel element. The optical fibres are aligned, at their input ends, with individual pixel elements of the panel display, so that the pixel structure of the display is carried over to the output plane of the image expander. Other image guides formed in this way may translate the image to provide a border-less abutment between a pair of adjacent panels.
A problem which occurs in this type of arrangement will now be described. In a display such as a backlit liquid crystal display, illumination is provided by light-emitting elements (e.g. LEDs, fluorescent tubes) behind the LCD panel. The light passes through LCD pixel elements which control the brightness and colour to be displayed at that pixel position. From there, the light passes into the image guide. If the backlight is incident on the pixel elements at greater than a certain angle, part of the output of each pixel may in fact pass into a part of the image guide intended to carry light from an adjacent pixel. This pixel cross-talk could cause a reduction in spatial resolution and contrast at the output (viewing) plane.
So to reduce the problem of cross-talk, it has been proposed that a collimated backlight arrangement should be used. An example of such an arrangement would provide a light source (e.g. a spaced array of white LEDs), the light from which is homogenised by a crossed pair of “Brightness Enhancing Films” manufactured by 3M Corporation and also passed through a collimator comprising an array of fine tubes or channels oriented along a direction between the light source and the display panel. The tubes have inner surfaces made of or coated with a suitable material such that light striking the inner surface is substantially absorbed. In common with most collimators, this arrangement works by attenuating light outside a defined range of incident angles.
A disadvantage of such an arrangement is that it provides an angular attenuation which is more severe than is actually required. To explain this point, reference will be made to FIGS. 7 and 8 of the accompanying drawings.
FIG. 7 schematically illustrates an array of channels forming a collimator as described above. Each channel has a length l and a width (a diameter in the case of a cylindrical channel) w. As drawn, incident light reaches the channels from the bottom of the page and collimated light is output towards the top of the page.
The angle of incidence is defined in the sense shown in FIG. 7. The minimum angle which can be passed, θmin, is defined by tan(θmin)=l/w. Light at angles of incidence less than θmin is completely attenuated (assuming an ideal absorption at the inner surface of each channel).
However, some light incident at angles greater than θmin is also attenuated. FIG. 7 illustrates light incident at θ1, greater than θmin. Because the light enters the channel at a position displaced from the edge of the channel, it still strikes the inner, absorbing, wall of the channel and is attenuated. So, θmin is the minimum angle of incidence at which light entering at the very edge of the channel is passed. For angles of incidence greater than θmin, the attenuation is also a function of lateral position of entry across the channel.
This attenuation profile is schematically illustrated in FIG. 8, which shows an example angular distribution of the source light and of the collimated light at the output of the collimator of FIG. 7, for angles of incidence decreasing from 90° (normal incidence), to θmin and then below.